The elucidation of host-virus interactions during retrovirus assembly will be the major focus of the research proposed. Mason-Pfizer monkey virus (M-PMV) provides a unique system for such studies in that the processes of capsid assembly, intracellular transport and budding/release are spatially and temporally separated, and the assembly of immature capsids within the cytoplasm allows the subcellular location of transport or budding defective structures to be visualized. It remains the system of choice for such studies since this dissection of the mechanisms involved in capsid assembly, intracellular protein transport and membrane extrusion is more easily approached than with a virus, such as HIV that simultaneously assembles and buds from the plasma membrane. Specifically we will: 1. Define the interaction of Gag and Gag-translating polysomes with components of the dynein motor machinery of the cell, using both cell-based and structural approaches. 2. Determine the role of viral and cellular components in the transport of capsids from the site of assembly to the plasma membrane. We will focus on the requirement for viral glycoproteins and the function of components of the endocytic recycling pathway. 3. Define the interactions of capsids with the vacuolar budding machinery of the cell, in order to understand how and where this complex is recruited by the virus. 4. Investigate the myristyl switch mechanism that is involved in virus budding and the structural changes imposed by myristylation of the Gag precursor. The approaches described above will extend our understanding of the molecular events involved in retrovirus assembly and will pave the way for rational approaches to develop therapeutics for other retroviruses that can interfere with this key event in the virus life cycle.